Abnormalities of chromosome 1q21 are common in B cell malignancies, including B cell lymphoma and myeloma, but the genes targeted by these aberrations are largely unknown. By cloning the breakpoints of a t(1;14)(q21;q32) chromosomal translocation in a myeloma cell line, we have identified two novel genes, IRTA1 and IRTA2, encoding cell surface receptors with homologies to the Fc and Inhibitory Receptor families. Both genes are normally expressed in mature B cells, but with different distributions in peripheral lymphoid organs: IRTA1 is expressed in marginal zone B cells, while IRTA2 is also expressed in germinal center centrocytes and in immunoblasts. As the result of the t(1;14) translocation, the IRTA1 signal peptide is fused to the Immunoglobulin Cα domain to produce a chimaeric IRTA1/Ca fusion protein. In Multiple Myeloma and Burkitt lymphoma cell lines with 1q21 abnormalities, IRTA2 expression is deregulated. Thus, IRTA1 and IRTA2 are novel immunoreceptors with a potentially important role in B cell development and lymphomagenesis.
B-cell Non-Hodgkin's Lymphoma (B-NHL) and Multiple Myeloma (MM) represent a heterogeneous group of malignancies derived from mature B cells with phenotypes corresponding to pre-Germinal Center (GC) (mantle cell), GC (follicular, diffuse large cell, Burkitt's), or post-GC B cells (MM) (for review, Gaidano and Dalla-Favera, 1997; Koppers et al., 1999). Insights into the pathogenesis of these malignancies have been gained by the identification of recurrent clonal chromosomal abnormalities characteristic for specific disease subtypes. The common consequence of these translocations is the transcriptional deregulation of protooncogenes by their juxtaposition to heterologous transcriptional regulatory elements located in the partner chromosome (Gaidano and Dalla-Favera, 1997). These heterologous transcriptional regulatory elements can be derived from the Immunoglobulin (IG) locus or from other partner chromosomal loci. Examples include MYC in t(8;14)(q24;q32) in Burkitt's lymphoma (BL) (Dalla-Favera et al., 1982; Taub et al., 1982), the CCND1 gene deregulated by the t(11;14)(q13;q32) in mantle cell lymphoma (MCL) (Rosenberg et al., 1991) and multiple myeloma (MM) (Ronchetti et al., 1999), BCL2 involved in the t(14;18)(q32;q21) in follicular lymphoma (FL) (Bakhshi et al., 1985), BCL6 in t(3;14)(q27;q32) in diffuse large B cell lymphoma (DLCL) (Ye et al., 1993), as well as FGFR3 in t(4;14)(p16;q32) (Chesi at al., 1997), MAF in t(14;16)(q32;q23) (Chesi et al., 1998) and MUM1/IRF4 in t(6;14) (p25;q32) (Iida et al., 1997) in multiple myeloma (MM). The identification of these oncogenes has offered valuable insights into the pathogenesis and diagnosis of their corresponding malignancies.
Chromosomal abnormalities involving band 1q21-q23 are among the most frequent genetic lesions in both B-NHL and MM. Among NHL subtypes, translocation breakpoints at 1q21-q23, including translocations and duplications, have been reported, often as the single chromosomal abnormality, in 17-20% of follicular and diffuse large B-cell lymphoma (DLCL), in 39% of marginal-zone B cell lymphoma (Offit et al., 1991; Whang-Peng et al., 1995; Cigudosa et al., 1999) and in 27-38% of Burkitt lymphoma, where they represent the second most common cytogenetic abnormality after translocations involving the MYC proto-oncogene (Berger and Bernheim, 1985; Kornblau et al., 1991). Comparative genome hybridization (CGH) has also identified 1q21-q23 as a recurring site for high-level amplification in 10% of DLCL cases (Rao et al., 1998). In MM, trisomy of the 1q21-q32 region has been reported in 20-31% of cases (Sawyer et al., 1995), amplification of the 1q12-qter region in 80% of cell lines and 40% of primary tumors (Avet-Loiseau et al., 1997), and nonrandom unbalanced whole-arm translocations of 1q, associated with the multiduplication of the adjacent 1q21-22 region, were found in 23% of patients with abnormal karyotypes (Sawyer et al., 1998).
The high frequency of involvement of 1q21 structural rearrangements in B-cell malignancies suggests that this locus may harbor genes critical to the pathogenesis of these diseases. Cloning of a t(1;14)(q21;q32) in a pre-B cell acute lymphoblastic leukemia cell, line previously identified a novel gene, BCL9 deregulated in this single case (Willis et al., 1998), but not involved in other cases. A recent report characterized the t(1;22) (q22;q11) in a follicular lymphoma (FL) cell line and found that the FCGR2B locus, encoding the low affinity IgG Fc receptor FCGRIIB, was targeted in this cell line and in two additional FL cases (Callanan et al., 2000). Finally, the MUC1 locus has been identified in proximity of the breakpoint of a t(1;14)(q21;q32) in NHL (Dyomin et al., 2000; Gilles et al., 2000), and MUC1 locus rearrangements have been found in 6% of NHL with 1q21 abnormalities (Dyomin et al., 2000). These results highlight the heterogeneity of the 1q21 breakpoints and the need to identify additional candidate oncogenes situated in this locus, since the large majority of these alterations remain unexplained.
The aim of this study was to further explore the architecture of 1q21 chromosomal rearrangements in B cell malignancy. To that end, we have employed a molecular cloning approach of the t(1;14)(q21;q32) present in the myeloma cell line FR4. We have identified two novel genes that are differentially targeted by 1q21 abnormalities. These genes code for five novel members of the immunoglobulin receptor family, IRTA1, IRTA2, IRTA3, IRTA4 and IRTA5 (Immunoglobulin superfamily Receptor Translocation Associated genes 1, 2, 3, 4, and 5), which may be important for normal lymphocyte function and B cell malignancy.